1. Field of the Invention
The present invention relates to a control system for an automatic transmission of the type having a planetary gear train and hydraulically actuated brake and clutch elements that selectively hold components of the gearset to produce the various drive ratios of the transmission. More particularly, the present invention relates to a control system that senses an idle or coasting condition in a forward drive range and automatically disengages a brake or clutch element thereby placing the transmission in neutral and removing load from the engine.
2. Description of the Prior Art
Hydraulically actuated, mechanically driven automatic transmissions generally include a turbine driven hydrodynamically from the impeller of a torque converter, the turbine transmitting power to the gearset to produce the forward drive ratios. When any forward or reverse gear is selected and though the vehicle is coasting or standing at idle, the engine transmits torque through the torque converter, the various clutches and gears to the drive wheels. In the idle condition, this causes the vehicle to creep forward, although the accelerator pedal is released, against the action of the service brake which must be activated to keep the vehicle stationary. Normally, the gear selector must be moved to the neutral position to disengage the forward gear.
It is preferable that the transmission be automatically shifted to the neutral position from the forward gear position whenever engine power is not required during the idle and coasting condition. The engine idle system can then be calibrated to improve fuel economy by reducing the torque transmitted through the automatic transmission.
Various hydraulic automatic transmission control systems have been devised such that a brake element is disengaged depending upon the setting of the gear selector, the application of accelerator pedal and the partial application of the brake pedal. For example, U.S. Pat. No. 4,298,109 describes a system for selectively interrupting the supply of pressurized fluid to a gear shift element of the transmission when the accelerator pedal is released and the brake pedal is depressed. The system is entirely hydraulically actuated and the logic is implemented through the use of control valves that open and close two sources of pressurized fluid to the gear shift element.
U.S. Pat. No. 4,105,101 describes a control system for automatically disengaging a brake element of a transmission operating in the lowest speed range in which a servo device is disengaged depending upon the actuation of the service brake. When the brake is actuated the transmission is temporarily shifted from the lowest gear ratio to neutral so that vehicle creeping is eliminated. This patent describes a brake-operated low gear ratio release system for an automatic transmission equipped with a mechanism for raising the idling rotational speed of the engine during warm up. A servo that selectively engages and disengages a gear ratio element of the gearset is automatically disengaged on actuation of the vehicle brake provided the transmission is operating in the lowest speed ratio.
It is preferable that a control system for automatically shifting a transmission to neutral not rely on the operative condition of the vehicle service brake. Instead, it is better that the control system perform its function depending on the road speed of the vehicle and the speed of certain drive shafts in the transmission, which give an accurate indication of the current torque requirement that should be placed on the engine.